DIN EN 12464 requires checking the illuminance on the main surfaces of enclosed areas. You can generate these with this tool. For empty rooms, it is a good idea to have them placed automatically. To obtain calculation results for the walls, floor and ceiling of a room, simply use the "Automatically select room main surfaces" function.
If furniture or obstructions cover the walls, it makes sense to create the main surfaces manually. Simply draw the contour of the relevant area as a rectangle or as a polygon. DIALux automatically creates suitable calculation objects. The easiest way is to construct the room main surfaces in the 2D view. Please note that a grid will not be generated for very small surfaces if the wall zones are larger than the surface itself.